An estimated 20 million tons of aluminum is produced each year by electrochemical smelting of aluminum oxide. The smelting operation is typically carried out in reduction cells, using a semi-continuous process. Aluminum oxide is dissolved in a molten cryolite salt bath, where it is reduced to aluminum metal and oxygen by electrolysis. The molten metal sinks to the bottom of the cell, and is periodically removed by siphon.
Electrical power is supplied to the cell by anodes, which are connected above the cell to a buss bar. The anodes are inserted into the molten bath of cryolite and aluminum oxide, and the provision of current from their surface results in the electrolysis reaction on their surface. Current collection typically occurs along the bottom surface of the cell, through conductive floor materials. Imbedded deep within these materials are iron collector bars, which extend through the outer shell of the vessel and complete the electrical circuit. The molten aluminum metal pool on top of the current collector provides the cathodic surface, and thus is an integral part of the electrical circuit. Maintenance of a continuous molten aluminum surface is therefore important to the efficient operation of the cell. It should be noted that other metals can be produced in this manner, including, in particular, magnesium.
Reduction cells are constructed primarily out of carbon-based refractory materials and are designed to last for 5-7 years. Historically, carbon is also used for both the anode and current collector materials. Pressed and fired carbon blocks are used for the anodes, as they provide suitable electrical conductivity and chemical stability against the molten reactants. However, oxygen driven off of in the electrolysis reaction reacts with the carbon anode to form CO.sub.2 gas, which must be removed safely from the system. Over 2/3 lb. of carbon are consumed for each pound of aluminum formed, resulting in more than 1.5 lbs of CO.sub.2 evolution into the environment. The total, worldwide production and subsequent release of CO.sub.2 into the environment due to this process is in the billions of tons annually. Additionally, the consumption of the anode by oxygen results in the requirement for frequent, periodic replacement of the anodes. Most aluminum smelting plants require an on-site plant dedicated to the continuous production of carbon anodes to satisfy the continual need for these components. The anode production method also contributes to release of pollutants, including CO.sub.2 as well as various toxic organic and metallic materials.
Carbon current collector material is produced by extrusion and firing. The "cathode" block carbon is mated to the iron buss bars, then inserted into the base of the reaction cell. Because of their location, these carbon materials are not generally consumed during cell use and do not need in-service replacement. However, over time, various factors resulting from the operation of the cell may cause loss of this material by erosion or corrosion. Eventually, regions on this cathodic surface erode to the iron buss bars, which are quickly consumed by aluminum metal. The result is loss of molten metal and cryolite through the floor of the cell through the consumed buss bar. This "tap out" of the cell is one of two most predominant reasons for cell shut down and replacement. While "tap out" of a cell may not occur until the cell has operated about 5 years, the vast number of cells in an operating smelter requires replacement and installation of new cells on a continuous basis.
Hall-Heroult materials are typically carbon based and are specifically chosen for where they will operate in the cell. Carbon generally meets most of the requirements, but specific types of carbon are chosen based on density, conductivity and purity. The materials used in the structural components of Hall-Heroult cells have not changed much since their initial invention. The existing materials limit the useful life, and performance of the cells and therefore the impact the final economics. Huge potential improvements exist in terms of energy consumption, process optimization and pollution control, which are limited by the available choice of materials.
The general requirements for improved materials for the electrodes in Hall cell application include:
Anode Material Cathode Material Electrical conductivity (critical) Electrical conductivity (critical) Inertness to aluminum and cryolite Inertness to A1 and cryolite Oxidation resistance (critical) Oxidation resistance Thermal shock resistance Erosion resistance Mechanical strength Wetted by Al metal (critical)
Due to their application, it is clear that both the anode and cathode materials need to be electrically conductive, as well as inert or resistant to the chemistry of the reactant system. Many of the other required properties are the same for both applications, but their priority is somewhat different. In the case of the anode, where oxygen is being produced at temperatures of nearly 1000.degree. C., the oxidation resistance of the material is of greatest significance. For the cathode, complete coverage by the molten Al metal (also referred to as "pad") is critical to provide the best electrical efficiency. Thus, Al wettability of the surface is of highest priority.
Interestingly, carbon is used for both applications, however it does not meet the requirements of the most critical parameters for either. In both cases, allowances are made to force carbon to work, as carbon has historically been the best and lowest cost material to cover the greatest portion of the requirements. In the case of anodes, oxidation of the carbon to CO.sub.2 is allowed and compensated for by continuous replacement of the anodes. For cathodes, poor wettability by aluminum is compensated for by use of a thick metal pad of several inches. These compromises have served the industry well throughout its early and middle years. However, new constraints are forcing a reevaluation of these allowances.
In the case of the carbon anode, issues with pollution are most critical. The continuous production of huge numbers of anodes contributes to release of hydrocarbons, pitches and tars, and metallic species into the environment. Later, during operation, the consumption of the anode releases massive amounts of CO.sub.2 into the air. Production of CO.sub.2 is estimated at more than 200,000 tons per year per smelter. While oxidation of the anode to CO.sub.2 occurs during the reduction process and provides heat energy to the system, it is not a required reaction in the metal reduction process. Use of a carbon-free material in this application would virtually eliminate production of CO.sub.2, resulting instead in production of oxygen. Thus, a significant pollution source would be eliminated. Furthermore, use of the significant petroleum and power resources required to make carbon anodes would be greatly diminished, which would have secondary impact on production costs as well as vital resource consumption.
For carbon cathodes, the issue of wettability requires a compromise in metal pad thickness. In order to force cathode coverage, metal pads of several inches thickness or more are used. The anodes are placed in proximity to the top of the metal pad, and adjusted to maintain a certain distance (referred to as the anode-cathode distance, or "ACD"). The thicker the ACD, the less electrically efficient the cell, so maintaining the ACD at minimum level is economically important. Unfortunately, the thick metal pad forces the ACD to be greater than desired. When sufficient metal accumulates, magnetic forces in the cell (caused by the significant current flow) create currents and waves in the metal pad. Should the unstable pad surface come in contact with the anode, the cell will short and become difficult to maintain in steady state. Therefore, typical cell operations require ACD sufficiently large enough to compensate for these conditions. A cathode exhibiting aluminum wettability would not require the thick metal pad, and thus magnetic irregularities would be greatly diminished. Electrode ACD could then be significantly reduced without concern of shorting, and consequently, significant energy savings (on the order of 20%+) could be realized. The monetary value of this energy savings is in excess of hundreds of millions of dollars annually, and also has obvious impact on the requirement for valuable energy providing resources. Cell designs that incorporate wettable cathodes typically use sumps to control the metal pad to minimal levels, and thus are termed "drained" cells.
The investigation into improved materials for non-carbon anodes and cathodes is as old as the technology itself. Obtaining materials with suitable electrical and chemical resistance properties to operate successfully for extended periods in the cell environment has proven a formidable task. To date, while a few materials have shown some promise, the overall performance and value/cost ratio of these technologies has limited their widespread introduction into the market.
For example, TiB2 has been examined for the Hall cell cathode application. While it generally provides the technical requirements for improved cathodes, the material exhibits a degree of solubility in cryolite salt over long periods of exposure that put into question the long term viability of the material. Despite many years of study, and even development of supportive cell operation procedures, the viability of this material for cathode applications has not been definitively established.